H. Grew — Period of Rotation of the Sun. 205 



tion period of the same part of the sun as that given by 

 magnetic variations. In this case the three particularly active 

 solar meridians, discovered by Braun, would appear to be essen- 

 tially regions of great thermal, rather than magnetic, activity. 

 That these variations are due primarily to the direct heating 

 effect of the sun is the basis on which Faraday, Christie, De la 

 Rive, and Stokes have each with varying success explained the 

 subject of terrestrial magnetism. 



But if the rotation jDeriod is to be determined from barome- 

 tric and magnetic disturbances, then regions of special solar ac- 

 tivity must be assumed to be persistent. Evidence of this from 

 other sources is doubtful, though the observations of Sporer on 

 sun spots and Wilsing on faculse would seem to indicate that 

 these outbursts have a tendency to recur at the same place 



Doppler's principle has already been applied by Zollner, 

 Vogel, and Young, to the determination of the relative veloci- 

 ties of the extremities of a solar diameter. Here we have under 

 investigation probably a different part of our luminary, viz : the 

 seat of selective absorption for the Fraunhofer lines, a layer of 

 considerable thickness mixed up with and extending into the 

 photosphere. So that this method probably gives the period of 

 a region higher than that of the sun spots. 



Later, Wilsing,* of Potsdam, has measured, by means of pho- 

 tography, the daily angular motion of a large number (1012) of 

 persistent faculse. Since these are seen as elevations in passing 

 over the sun's limb and give brighter spectraf than surrounding 

 portions of the solar surface, there is reason for thinking that 

 the period thus obtained by "Wilsing is that of a region some dis- 

 tance above the photosphere. 



There are, then, at least four distinct solutions of the problem 

 in hand : 



{a) The observation of sun spots, giving the period of the 

 lower parts of the photosphere for various latitudes. The data 

 in our possession are here limited to a zone extending a little 

 less than 45° on each side of the equator. 



(b) The observation of the meteorological (including mag- 

 netic) elements, giving the period of an unknown region, possi- 

 bly of the more solid interior, possibly of the region of the 

 great uprushes around the spots. 



(c) The measurement of the difference of refrangibility of 

 light coming from the eastern and western limbs of the sun, 

 suggested by Zollner. 



Since the accuracy of this method varies as the cosine of the 

 latitude, observations are, at present, of no value for parallels 

 higher than 75°. 



* Wilsing, Publicationen des Astrophysikalischen Observatoriums, 1884. 

 t Lockyer, Chemistry of the Sua, p. 105. 



